Field of the Invention
The present invention relates to an antireflection film and an optical member, having excellent oil contamination resistance, and relates to a method of producing an optical member.
Description of the Related Art
In order to prevent the reflection of light at incident or emitting surfaces of optical members, antireflection films each having a thickness of several tens to several hundred nanometers and being composed of a plurality of monolayer or multilayer optical films having different refractive indices have been used. These antireflection films are produced by a vacuum deposition process, such as vapor deposition or sputtering, or a wet film-formation process, such as dip coating or spin coating.
The outermost layer of such an antireflection film is made of a transparent material having a low refractive index, i.e., an inorganic material, such as silica, magnesium fluoride, or calcium fluoride, or an organic material, such as a silicone resin or an amorphous fluororesin.
In order to further reduce the reflectivity, a low refraction film utilizing the fact that air has a refractive index of 1.0 has been recently used as the antireflection film. The refractive index is reduced by forming voids in a layer of silica or magnesium fluoride. For example, the refractive index of a magnesium fluoride thin film can be reduced from 1.38 to 1.27 by forming 30 vol % of voids in the thin film.
The voids can be formed by, for example, utilizing hollow silica particles. An antireflection film having a low refractive index is prepared by using the hollow silica particles. (Japanese Patent Laid-Open No. 2001-233611).
Alternatively, an antireflection film having a low refractive index can be prepared by forming voids between fine particles. This process, however, has a risk of invasion of contamination into the voids between the fine particles. If the contamination is oily matter, the contaminants are hardly removed, resulting in a reduction in the antireflection performance (Japanese Patent Laid-Open No. 2004-258267).
Although the removal of the oily matter from the antireflection film is took into account in the process described in Japanese Patent Laid-Open No. 2004-258267, the diffusion of the oily matter is not took into account. In the process described in Japanese Patent Laid-Open No. 2004-258267, in order to enhance the removal of oily matter, a coating solution prepared by adding an oil repellent component to a binder coating solution is given to the voids between particles. In the finally resulting layer, the binder component consequently contains a fluorine component, resulting in that a high oil repellent property is not expressed.
In addition, in the antireflection film described in Japanese Patent Laid-Open No. 2004-258267, since the fluorine component is added to the binder coating solution, an increase in the amount of fluorine for obtaining an oil repellent property reduces the strength of the film.